This invention relates to an electromagnetic valve, and in particular to an electromagnetic valve of the type used in a hydraulic control system.
Japanese Utility Model Laid-Open Nos. 61-82185 and 61-52776 disclose examples of conventional electromagnetic valves used in a hydraulic control system, which are shown in FIGS. 4 and 5, respectively.
The electromagnetic valve shown in FIG. 4 includes a casing 30 having in it sleeves 34 in which plungers 31 are slidably arranged, the plungers 31 being driven by means of solenoids 32 provided outside the casing 30. The plungers 31 are equipped with respective ball valves 33 which are adapted to abut against respective valve seats 35, thereby effecting a valve action.
In this type of electromagnetic valve, it is necessary to provide in the plungers 31 themselves or between the plungers 31 and the casing 30 a liquid-flow path allowing the working liquid to pass through the valve. In the example shown, bias springs 36 arranged between the respective plungers 31 and the sleeves 34 provide such a flow path.
In the electromagnetic valve shown in FIG. 5, which is of substantially the same type as the one shown in FIG. 4, the requisite flow path is provided by grooves 40 formed on the side surface of the plunger 31 as well as by a hole section at the center of the plunger 31.
The problem with these prior art structures is that the working liquid is allowed to flow over the side surface of the plunger as well as through the central section thereof, which adversely affects the movement of the plunger, thereby making unreliable the action of the plunger in response to control signals. In other words, these conventional electromagnetic valves lack in responsiveness. As a result, these valves must have quite a large size before they can provide an operational force which can overcome the resistance of the working liquid.
The movement of the plunger might be made smoother by enlarging the plunger side surface as well as the flow path at the center thereof, which would also help to prevent the working liquid from adversely affecting the plunger movement. However, this will inevitably involve an enlargement in the valve size before a sufficiently wide flow path can be secured.